The machining of recesses having angled side walls in metal workpieces to close dimensional tolerances is a practical nightmare to machinists and tool and die makers using present-day measuring devices. For example, a machinist may be required to machine a cavity 0.500 inch deep with a side wall rising at a 45.degree. angle from a flat bottom wall of the cavity to a dimension or diameter of, say, 5 inches .+-. 0.001 inch across the inside corners of the cavity. This tolerance is extremely difficult to obtain using conventional measuring tools.
As the workpiece is taken from the machine, with the machinist's great hope that the dimensions of the cavity are correct to specification, the piece is then taken to the inspection room and placed on a $30,000 to $40,000 comparator to check the critical dimension. The machinist soon learns, in a practical case, that he is 0.003 or 0.005 over or under the required dimension. If the cavity diameter or width is undersized, he will simply place it back in the machine, indicate carefully within 0.0005 inch an attempt to machine 0.003 or 0.005 inch off of the angled side wall and then start the whole measuring procedure over again. If the cavity proved to be oversized, the machinist will be compelled to start all over again with a new workpiece. This hit-and-miss machining and measuring procedure is very time-consuming, inconvenient and generally inefficient and costly.
An alternate procedure is available in the prior art, probably a bit more satisfactory than the above. In the alternate procedure, the machinist will first make a male plug to the shape and size of the cavity. He will then try to machine the required cavity in the workpiece until the male plug fits exactly all around the cavity side wall and on the bottom. This of course is a lengthy, tedious and expensive procedure.
The invention has for its objective to completely eliminate the above problem or difficulty involved in the precision machining of cavities having angled side walls in workpieces, regardless of whether the side wall angle is open or inverted in the workpiece. The device of the invention also possesses the ability to gage or measure with high accuracy the depth of narrow machined grooves, both internal and external.
The objective of the invention is realized by providing a caliper-type measuring device having an efficient leg spreading and retracting mechanism and with the legs of the instrument equipped with readily interchangeable ball elements for measuring machined recesses and narrow right angular elements for use in measuring narrow machined grooves.
More particularly, the caliper measuring device utilizes electrical sensing or contact elements on one or both legs thereof electrically connected through such leg or legs with light emitting elements conveniently housed within compartments formed in the two legs of the device. In the embodiment of the invention used to make a precision measurement between the side walls of a machined recess, an interior central contact pin is engaged electrically with the bottom of a machined cavity while a surrounding exterior ball contact element is brought into tangent contact with the angled side wall of the cavity. The opposite leg of the caliper device may then serve as a ground for the device. In an alternate arrangement, the caliper device is grounded through a separate strap element and both legs are equipped with the electrical sensing elements which must be properly located with respect to the cavity surfaces to light up four light emitters simultaneously to assure the proper measurement.
The proper use of the invention assures high precision in the measurement of machined cavities and grooves. When the light emitters are energized, there is no doubt or guesswork on the part of the machinist. The electrical sensing contacts are far more sensitive than the human hands. By the use of the invention, a critical cavity measurement can easily be obtained within a tolerance of 0.0002 inch. Even where batteries are dead or diodes burned out, the device can still be used effectively by applying spotting blue on opposite sides of the adjustable ball element. By moving the ball element left and right across the center and touching the blued section, lines will appear on the bottom of the cavity and side wall. When these lines are one above the other, the desired measurement is achieved.
Some efforts have been made in the prior art to devise electrified calipers but none of these efforts has resulted in providing a device which even approaches the precision capabilities of the present invention. The following prior U.S. Pat. Nos. represent the known prior art and are made of record herein under 37 C.F.R. 1.56, duty to disclose: 611,391; 1,816,464; 2,401,962; 2,445,068.